Once upon a time a boy was walking through a little forest when he found a big blackboard hanging on an old tree trunk. It came complete with a box of chalk pieces, and bright sparks were flying from the tips of every piece of chalk. The boy took a piece of chalk and started to draw on the board: first a tree, then a rabbit, then a flower…magically, whenever he finished a figure it came to life and jumped out of the board. Before long, the place had become a wonderfully green and fertile forest, full of animals happily playing. Excited, the boy also drew his parents and brothers and sisters enjoying a picnic, with their sandwiches and cold meats. He also drew the paper plates and the empty tins of sardines left on the ground, which always happened on their picnics. But when these bits of litter came to life something awful happened: around every paper plate and every empty tin, the forest turned grey, and this greyness started quickly spreading everywhere: to the grass, to the flowers, to the animals… the boy realised the litter was to blame for all the greyness, so he ran through the forest with the blackboard rubber in his hand to rub them out wherever they had landed. He was lucky. He was quick, and didn't leave a single piece of litter. The forest and its animals managed to recover, and they played together for the rest of the day. The boy never again saw that blackboard, but now, every time he goes to the countryside with his family, he remembers that adventure and is the first to pick up any litter. And he is sure to remind the others that anything left lying in the woods can do more damage to the animals than they ever would have suspected. 
Some things you can't leave in the countryside because they damage the environment and living creatures. 